1. Field of the Invention
This invention relates to a quality control system for an irradiation apparatus, and more particularly to a quality control system for checking a position exposed to the radiation in an irradiation apparatus.
2. Description of the Related Art
There have been known a radiotherapy system in which a lesion is treated by concentrating radiation on the lesion, as disclosed, for instance, in Japanese Utility Model Publication No. 2593412 and “Zukai Shinryo-hoshasen Gijutu Jissen Gaido (Illustrated Radiodiagnosis Practice Guide)” (2002) Kabushiki Gaisha Kobundo. In such a system, after the position of the lesion is adjusted so that the lesion is on the optical path of a position check light which is arranged to propagate along the same optical path as the treatment radiation, the treatment radiation is applied to the lesion, whereby the treatment radiation is accurately applied to the lesion.
The tolerance of the irradiating position (the position exposed to the radiation) is within several millimeters, and whether deviation of the optical path of the position check light from the optical path of the treatment radiation (will be simply referred to as “the optical path deviation”, hereinbelow) is within the tolerance where the deviation does not cause any trouble in the radiotherapy is checked each time the radiotherapy system is operated. Whether the optical path deviation is within the tolerance can be checked, for instance, by preparing an X-ray film with a central hole, visually positioning the X-ray film so that the central hole is aligned with the center of the bundle of the position check light, applying to the X-ray film position check radiation which propagates along the optical path identical to that of the treatment radiation, and reading the relation between the position of the central hole and the position in which the X-ray film is exposed to the position check radiation.
In order to ensure that the optical path deviation is within several millimeters, it is desired that the accuracy of measurement of the optical path deviation is higher than the value by one figure. However, it is troublesome operation to accurately position the X-ray film so that the central hole is aligned with the center of the bundle of the position check light, whereby the efficiency of operation of radiotherapy deteriorates and at the same time, the reliability of measurement of the optical path deviation becomes unsatisfactory.